US Secretary of State John Kerry has called his Russian counterpart Sergei Lavrov and expressed his “strong concern” over the collapse of peace talks between Moscow-backed separatists and the Ukraine government.

During the call Kerry “expressed strong concern about the refusal of Russian-supported separatists to take the necessary steps and provide the kinds of assurances that would have enabled an extension of the ceasefire and stressed the importance of taking steps to de-escalate tension,” a State Department spokesperson said.

Kerry also made clear that the US and its European and international partners will continue to press Russia to “call on separatists to lay down their arms, to return the border checkpoints they hold to Ukrainian government control, and to release all remaining hostages.”

Ukraine and its Western allies have accused Russia of helping to arm and fund the separatists in reprisal for the February ouster of a Kremlin-backed leader who had rejected closer European ties. Russia denies those claims but still faces the threat of sanctions against its financial and energy sectors.

The two leaders also discussed the P5+1 negotiation that will resume in Iran later this week and the grave security situation on the ground in Iraq, the spokesperson said. Later, Deputy National Security Adviser Ben Rhodes, told foreign journalists that the US very much wants to work with Russia where we can.

“More broadly, we have been cooperating with Russia through the P5+1. They have an important role to play in that process and insisting that nonproliferation is upheld as a fundamental international norm. “And so if you look at the fact that we will be at the table of this negotiation, it’s important that Russia stand with the rest of the international community in insisting that Iran meet its obligations,” Rhodes said.

“So there are going to be areas of cooperation between the United States and Russia, but we are going to have differences.I think those differences have grown, obviously most acute over Ukraine. But they are rooted not in any desire by the United States to seek out punishment for Russia,” he said.

The White House official said people should have the ability to make determinations about their own future, who they want to associate with. “That’s what has guided our Ukraine policy throughout this whole process,” Rhodes said.